The boil on the buttock along with pus are indicative of an underlying local infection. If you have over- weight issues or if your diabetic (with a family history of diabetes) it is more common. It needs to be evaluated by a dermatologist to assess need for drainage or if it can be managed just with oral and local anti biotics. Kindly get a consultation soon to prevent it from worsening and enlarging further. Hope this was helpful.